Field
The present disclosure relates to a technique for operating an optical source. More specifically, the present disclosure relates to a technique for tuning an external cavity laser.
Related Art
A hybrid III-V semiconductor-silicon laser is being investigated as a silicon-photonics light source. By fully utilizing the mature technology of III-V semiconductors and silicon-photonics independently, a most efficient and manufacturable light source can be realized using a silicon-photonics platform. In particular, a silicon-on-insulator (SOI) platform can provide wavelength selectivity and a semiconductor chip can provide optical gain.
Moreover, laser-wavelength tunability is becoming an important capability for a light source in optical communications. This is particularly the case in wavelength-division-multiplexing (WDM) applications in which the optical signals include multiple wavelengths. Therefore, in these applications a tunable laser source may provide flexibility and may eliminate the need for multiple single-wavelength lasers. Laser tunability is also important in allowing the laser wavelength to be matched to other components, such as a resonator-based modulator.
In a hybrid laser configuration, the laser-wavelength control circuit is typically implemented on the SOI chip. Moreover, because of their resonant nature (Lorentz-type line shape) and efficient wavelength-tuning mechanism (such as the thermo-optic effect and/or the free carrier dispersion), micro-ring resonators can provide excellent wavelength selectivity. For example, a micro-ring resonator in conjunction with a 1×2 splitter (or Y-junction) can form a loop-type reflector that feeds the resonance wavelength of the micro-ring resonator back to the optical cavity. Note that the optical bandwidth of the feedback signals is determined by the quality factor (Q factor) of the micro-ring resonator and can be controllable by adjusting the coupling coefficient of the micro-ring resonator.
While a single ring-resonator reflector typically provides excellent single-wavelength feedback, the tuning range is usually limited to one free-spectral range or FSR (e.g., a ring resonator with a 5 μm radius provides an FSR of approximately 20 nm). Consequently, in order to have a wide tuning range, the ring resonator typically needs to have a very small radius. For example, in order to have a tuning range of 40 nm, a single ring resonator typically has a radius less than 3 μm. However, the bending loss of a ring resonator increases significantly in this small bending-radius regime, and typical heaters (such as a metal heater or a silicon heater) become inefficient because of the short length. Furthermore, because the Q factor of the ring resonator is reduced (and, thus, the feedback filter is broadened), multiple optical cavity modes can lase within the ring resonance, which can adversely affect laser stability and can result in mode-hopping.
In principle, these issues can be addressed using a Vernier dual ring-resonator reflector. In particular, two micro-ring resonators with slightly different radii can provide an extended wavelength tuning range via the Vernier effect. For example, a 45 nm tuning range (i.e., FSR) may be obtained using ring resonators with radii of 7.5 μm and 10 μm. This wide FSR may facilitate stable and single-mode laser operation.
However, in practice, a Vernier dual ring-resonator reflector often poses additional challenges. For example, one of the challenges with a Vernier dual ring-resonator reflector is wavelength control, because it requires precise control of two independent ring resonators so that they remain aligned with each other, and so that only one reflection wavelength can pass through in the entire laser cavity. In particular, this requirement typically requires that the resonance band of each ring resonator be examined in advance, aligned with each other and confirmed in a spectral domain. This approach is usually expensive and slow, which can make a Vernier dual ring-resonator reflector less attractive.
Hence, what is needed is an optical source without the above-described problems.